Sample injector for gas chromatographs



1968 D. JENTZSCH ETAL 3,365,951

SAMPLE INJECTOR FOR GAS CHROMATOGRAPHS Filed April 12, 1965 DIETRICH JE NTZSCH WOLFGANG SCHUMANN INVENTORS United States Patent 3,365,951 SAMPLE INJECTOR FOR GAS CHROMATOGRAPHS Dietrich Jentzsch and Wolfgang Schumann, Uberlingen (Bodensee), Germany, assignors to Bodenseewerk Perkin-Elmer & Co. G.m.b.H., Uberlingen (Bodensee), Germany Filed Apr. 12, 1965, Ser. No. 447,255 Claims priority, application Germany, Apr. 24, 1964, B 76,492 9 Claims. (Cl. 73422) ABSTRACT OF THE DISCLOSURE A system for repetitive injection of liquid samples into, for example, the inlet of a gas chromatograph includes a connection from the liquid sample reservoir to the chromatograph inlet, the conventional carrier gas supply connection to this inlet, and an optionally operable means for temporarily reducing the carrier gas pressure at the chromatograph inlet. This optionally operable means includes a pressure-reducing restrictor or throttle between the carrier gas supply and the chromatograph inlet, and a normally open, but optionally closable by-pass valve, so that the restrictive throttle is normally by-passed or shunted. Optional closing of this valve temporarily reduces the carrier gas pressureat the inlet, thereby causing a certain portion of the liquid sample in the reservoir (which is at full normal carrier gas pressure) to be pushed into the inlet of the chromatograph. By using, for example, a solenoid-operated by-pass valve, the liquid sample injection may be readily repeated at any desired time intervals.

This invention relates to a sample injector for gas chromatographs, particularly adapted to provide periodic injections of liquid samples to the chromatographic separating column. The invention is especially suitable for use in so-called preparative chromatographic processes, in which the sample components are separated for the purpose of further analysis or other use thereof.

One form of existing apparatus for this purpose utilizes a sealed reservoir (holding the liquid sample), which is connected with a carrier gas line leading to the entrance of the separating column through a throttling or restrictive connection line, so that a pressure difference can be produced between the reservoir and the carrier gas line during desired predetermined periods of time. In one such known arrangement, a first carrier gas branch is maintained at a specific pressure (say, P by means of a pressure control; this first branch is connected to the entrance of the apparatus, for example, the sample evaporator or injection block of the gas chromatograph. A second branch from the carrier gas source also contains a pressure control, which is adjusted to yield a somewhat higher pressure, P (i.e., P is larger than P This second carrier gas branch is connected with the closed reservoir by means of a first solenoid valve. This reservoir contains a supply of the liquid sample to be analyzed and is also connected with the first carrier gas branch by means of a capillary tube. Additionally, the reservoir is connected to the atmosphere through a venting tube containing a second solenoid valve.

In such known apparatus, both solenoid valves are initially closed. Pure carrier gas therefore flows through the first branch (at a pressure P to the gas chromatograph. In order to inject some of the sample, the first solenoid valve is opened for a short period of time. This causes the carrier gas to flow into the reservoir with the higher pressure P (i.e., the injection pressure) so that the pressure difference, F -P forces the liquid sample Patented Jan. 30, 1968 through the capillary into the entrance of the apparatus. After the desired quantity of sample has been introduced, the first solenoid valve is closed. However, since the entire reservoir has been built up to the injection pressure (P this excess pressure at the liquid surface will push some of the sample in the reservoir through the capillary tube to the apparatus even after the first solenoid valve has been shut off. In order to avoid this excess flow of sample material, the second solenoid valve is opened simultaneously with the closing of the first solenoid valve, so that the excess pressure in the reservoir is released through the second solenoid valve (usually through a throttling element, such as a throttle valve). In this manner the sample injection through the capillary tube is terminated relatively sharply. After the excess pressure in the reservoir has been released, the second solenoid valve is again closed; and the entire cycle may then be repeated.

The type of sample injector just described is primarily utilized for preparative purposes (i.e., for separation of relatively large quantities of sample so as to allow subsequent use of the separated components). Therefore predetermined quantities of the sample are periodically injected into the gas chromatograph as long as there remains some sample in the reservoir. This known arrangement suffers from the following drawbacks:

(1) During the major portion of the operating period (that is, during the time that there is no sample injection), the solenoid valves are closed. For this reason any leaks in the valves may cause considerable errors in the sample actually injected.

(2) The solenoid valves are exposed to the vapors of the sample substance. This is particularly true of the second solenoid valve (i.e., the venting valve), where the sample liquid might deposit during the release of the excess pressure remaining in the reservoir after termination of the sample injection. Since definite specific partial pressures of the different sample components always exist above the surface of the sample in the reservoir, a certain amount of the sample components (as vapor)will always pass through the solenoid valves and therefore may cause corrosion or sticking thereof.

(3) Every time the excess pressure in the reservoir is released through the second solenoid valve, a certain quantity of sample substance (as vapor) is exhausted together with carrier gas. When the sample material is quite valuable, this is obviously an uneconomical waste of such material. In addition the composition of the remaining sample may be changed, since the more volatile components are lost at a greater rate than the less volatile components.

(4) There can be no sharp cut off of the sample injection, since the exhausting through the second solenoid must be accomplished through a throttle element. If this throttle element is not used, then an adiabatic cooling of the gas would occur, causing undesired fogging (i.e., condensation). The primary object: of the invention is to avoid the just-mentioned disadvantages of the prior known arrangement.

Generally this is accomplished by placing a pressure reducing throttle in the carrier gas line at a point before (i.e., more remote from) the gas chromatograph entrance than the connection of the sample reservoir. This pressure reducing throttle is bridged or shunted by a by-pass line, which may be opened or closed (by means, for example, of a solenoid valve). When the by-pass line is open, the full carrier gas pressure (say, P will be present at the entrance to the apparatus. This pressure will then be imparted to the closed reservoir through the connecting line thereto. In other words, the carrier gas will flow back into the reservoir through this connecting line until the pressure in the reservoir has been balanced with the pressure in the carrier gas line (i.e., the gas in the reservoir above the surface of the sample will also be at pressure P If the by-pass line is then shut, a pressure drop will develop across the throttle. This will cause the pressure in the carrier gas line at the entrance of the apparatus to become less than pressure P This reduction in the pressure will cause the pressure in the reservoir (which is still P to be greater than the pressure in the carrier gas line at the entrance of the apparatus. Consequently liquid from the reservoir will be pushed by this excess pressure into the entrance of the apparatus. Whenever the by-pass line is again opened, the pressure drop in the throttle is immediately eliminated and the full pressure P will again prevail in the entire carrier gas line. Now the pressure in the connecting line from the liquid reservoir will actually be lower than the pressure in the gas carrier line, since the pressure in the reservoir will have dropped somewhat below P Therefore the injection of the sample liquid from the reservoir will immediately terminate. The pressure in the reservoir will again build up to the value P by means of back flow of the carrier gas through the connecting line, until the necessary quantity of carrier gas has entered the reservoir to make up for the lost volume caused by the discharge quantity of sample liquid.

In the inventive arrangement outlined above, the solenoid valve for controlling the sample injection is in the carrier gas line exclusively. Therefore this solenoid valve cannot come into contact with either the sample liquid or vapors thereof, thereby obviating any possibility of the valve being attacked by potentially corrosive substances. Further, this solenoid valve is open during the major portion of the time that the apparatus is in operation (i.e., it is open except for the short time intervals during which the sample is being periodically injected). For this reason the solenoid valve is not being constantly strained by being biased to a closed position, as occurs in the prior art arrangement. An additional advantage is that the reservoir is maintained in a closed condition continuously, and there is no need to release or exhaust the carrier gas contained therein into the atmosphere periodically. This avoids both general and differential losses of sample components, and further prevents any damage to solenoid valves which may be caused by the exhausting sample vapors. Finally, the end of the sample injection action is effected substantially instantaneously as soon as the by-pass line (around the carrier gas throttle) is shut off.

The manner in which the above-mentioned disadvantages of the prior art are overcome by apparatus made according to the invention will become apparent from the following detailed description of the preferred embodiment of the invention in conjunction with the drawing, in which:

The sole figure is a schematic representation of the sampling injector for a gas chromatograph.

In the drawing carrier gas flows in the carrier gas line shown at the left-hand side of the drawing from a carrier gas source (not shown) through a fine pressure control, FPC, on its way to a gas chromatograph (not shown), situated beyond the arrow shown at the right-hand side of the drawing. After the carrier gas has been adjusted by the fine pressure control, it passes through a pressure sensor 2, the purpose of which will be subsequently explained. The gas then flows through a restrictor or throttle N The by-pass channel or line 1 is arranged in parallel shunting arrangement to the throttle N so as to provide a possible alternate path for the gas around this throttle. By-pass line 1, however, contains a solenoid valve M which may open and close the passage through by-pass channel 1. The pressure of the carrier gas (in front of the throttle N may be measured by means of a manometer 3. The sample liquid to be supplied to the gas chromatograph is accommodated in a reservoir 4,

which is closed in a gas-tight manner. A capillary tube K passes through the closure lid of reservoir 4, so as to be covered by the liquid sample. The upper end of capillary tube K joins into the carrier gas stream (at 5) near the entrance to the gas chromatograph. In reservoir 4, there exists above the sample liquid a mixture of carrier gas and vapors of the various (volatile) liquid sample components.

As long as the solenoid valve M remains in its normal opened condition, throttle N is ineffective; and the full controlled pressure (P of the carrier gas will be present in the right-hand side of the carrier gas line, such as at junction point 5. Should the pressure in reservoir vessel 4 be lower than this normal carrier gas pressure (P then carrier gas will flow through capillary tube K (and bubble through the sample liquid) until the pressure in reservoir 4 is in equilibrium with the pressure in the carrier gas line. Therefore at any time that solenoid valve M is open, the pressure at point 5 and that within reservoir vessel 4 will soon equilibrate at the normal carrier gas pressure, P

If the by-pass channel 1 is now closed (by actuating valve M then the carrier gas must flow through the restrictor or throttle N (which may comprise, for example, a restriction of the needle valve type). The restriction or resistance of the throttle produces a pressure drop, so that the pressure at point 5 decreases below the normal full carrier gas pressure, P Since, however, the pressure in reservoir 4 is still at the normal full pressure, P (or, more explicitly, the gas volume above the liquid surface in vessel 4 is at this pressure), the liquid in the reservoir is forced through the capillary tube K into the entrance of the gas chromatograph. After a small amount of liquid sample has thus been injected, solenoid valve M is again opened; and the original conditions are established once more almost immediately.

An extremely highly resistant throttle valve N may be provided in a separate outlet from the reservoir, communicating with the atmosphere. The purpose of this highly restrictive throttle N is to allow a very slow escape of gas under excess pressure caused by temperature variations. The resistance of this valve may be adjusted in such a manner that, during the intervals when no sample is being injected, the carrier gas must enter vessel 4 through capillary K as individual bubbles at spaced intervals, in order to balance the pressure as previously mentioned. In this Way, an unintentional injection of the sample liquid is precluded.

It is also possible that the pressure in the carrier gas line (such as at point 5) decreases below its nominal value (P even though solenoid valve M is open. This may occur because of a leak in the carrier gas line or because of a reduction in pressure of the carrier gas source (i.e., the gas cylinder supplying the carrier is almost empty). The reduction in pressure in the carrier gas line at 5 might cause an erroneous injection of sample liquid unless some provision is made to avoid this. For this .purpose the pressure sensor 2 (which may be sensitive either to absolute or differential pressures) is provided at the side of the fine pressure control (FPC) remote from the carrier gas source. A drop in pressure, as determined by sensor 2, will cause an additional outlet from the reservoir 4 to be opened to the atmosphere. Specifically sensor 2 will cause a second solenoid valve M to open allowing an escape of much of the pressure in reservoir 4 through a throttle N which throttle is of relatively small resistance. To insure that carrier gas will not keep bubbling through the liquid by flowing back through capillary tube K, a further solenoid valve M may be closed at the same time. In order to assure absolutely that an undesired sample injection cannot occur, it is also possible to provide an additional solenoid valve (similar to valve M in the capillary tube or else supply an additional control of solenoid Valve M This additional means would cause the capillary tube K to be closed whenever the by-pass solenoid valve M is open. Therefore a sample injection could occur only when control solenoid valve M is closed. Solenoid valve M may additionally be controlled by a mechanism (such as pressure sensor 2) in the carrier gas line, which mechanism would close the solenoid valve M (or another valve placed in capillary K) whenever the pressure from the carrier gas source dropped below a certain level (caused, for example, by a gas cylinder becoming empty, leaks in the various gas lines, or the like).

The by-pass or control solenoid valve M may be energized (i.e., closed) periodically by any suitable control device, generating an electrical control signal. In addition to the various optional or alternative features mentioned at the end of the foregoing specification, there are other alternatives and modifications which will be obvious to one skilled in the art. The invention is not intended to be limited therefore to the specific embodiment illustrated in the drawing and described hereinbefore, but rather is defined in the appended claims.

We claim:

1. A sample injection device, particularly suitable for the periodic injection of liquid samples into a gas chromatograph, comprising:

a carrier gas line, adapted to receive carrier gas from a source at a first point and to supply said carrier gas to the entrance of a gas chromatograph at a second point;

a closed, substantially gas-tight reservoir, adapted to contain the liquid sample to be injected into the gas chromatograph;

a relatively narrow tube connecting a sample point near the bottom of said reservoir to said carrier gas line at a junction point near said second point thereof;

a pressure-reducing restrictive throttle means positioned in said carrier gas line between said first and said second points, whereby a pressure drop in said carrier gas line will occur when gas in flowing through said throttle means;

a bypass line connected to said carrier gas line at opposite sides of said throttle means, so as to provide a shunting path for said carrier gas around said throttle;

and optionally operable valve means in said by-pass line, adapted to open and close said by-pass line;

whereby closing of said valve means in said by-pass line will cause a pressure drop to be created at said junction point, thereby causing any liquid sample in said reservoir to be pushed through said tube to the point near the entrance of said gas chromatograph, thereby injecting a portion of said sample.

2. The sample injection device according to claim 1, in

which:

said valve means in said by-pass line is a solenoid valve.

3. The sample injection device according to claim 1, in

which:

said relatively narrow connecting tube is a capillary.

4. The sample injection device according to claim 1, in

which:

a fine pressure control means is provided in said carrier gas line between said first point thereof and said throttle means.

5. The sample injection device according to claim 4, in

which:

an optionally openable venting outlet is provided for said reservoir;

and a pressure sensor means is positioned in said carrier gas line between said fine pressure control means and said throttle means,

said pressure sensor means being operatively connected to said venting outlet, so as to open the latter whenever the pressure in said carrier gas line at said sensor substantially decreases.

6. The sample injection device according to claim 5, in

which:

a shutofi valve is positioned in said narrow connection tube;

and said pressure sensor means is operatively connected to close said shutoff valve when said carrier gas pressure decreases,

whereby undesired injections of said sample are positively precluded if the pressure from said carrier gas source should decrease.

7. The sample injection device according to claim 1, in

which:

a shutoff valve is provided in said narrow connection tube;

and control means are provided for closing said shutoff valve during the time that the value in said bypass line is open.

8. The sample injection device according to claim 4, in

which:

a shutoff valve is provided in said narrow connection tube;

said shutolf valve being operatively connected to said pressure sensor means so as to be closed thereby upon the occurrence of a substantial decrease in the carrier gas line pressure.

9. The sample injection device according to claim 1, in

which:

a constant vent, including a very highly resistant restriction, is provided for said reservoir,

said restricted vent thereby allowing escape of gas therethrough so as to reduce excess pressure caused by temperature changes.

References Cited UNITED STATES PATENTS 3,103,807 9/1963 Brocrman 73-23.l

LOUIS R. PRINCE, Primary Examiner. S. C. SWISHER, Assistant Examiner, 

